Man I have no idea. It's going to be really hard to recreate water... hair.
Other than getting a wig that looks similar in color, the only other thing I can think of is somehow making it out of hollow, flexible, clear plastic.. and then stringing the fish in somewhere.

Or you can make some parts hair - some parts plastic interweaving it artistically and such..

EDIT: One suggestion I will give is to think of a character that also has water hair/parts, look of their cosplays, and see what others did for it. May give you some ideas.
Right now I can't think of any popular ones off the top of my head.